ATOMIC DATA AND SPECTRAL-LINE INTENSITIES FOR MG-VIII

EUV spectral lines of Mg vm have been observed from the solar corona b y Skylab, SERTS, and more recently by SOHO. Atomic data for this astro physically important ion are presented in this paper. The atomic data include energy levels, oscillator strengths, transition rates, and ele ctron-impact excitation collision strengths. Twenty levels are include d by using the configurations 2s(2)2p, 2s2p(2), 2p(3), 2s(2)3s, 2s(2)3 p, and 2s(2)3d. Collision strengths are calculated at five incident en ergies (15.0, 22.5, 30.0, 37.5, and 45.0 Ry) in the distorted wave app roximation. The temperature of maximum abundance for this ion is log T -e= 5.9 K. The rate coefficients are calculated at this temperature by averaging the collision strengths over a Maxwellian distribution. Pro ton rates and solar background radiation have been included in calcula ting level populations, and intensity ratios are calculated by includi ng proton rates only. The relative intensity ratios are compared with observed ratios obtained from SERTS and SOHO/CDS to infer electron den sity in different solar features such as an active region, quiet-Sun, and off-limb. With a few exceptions, the predicted intensity ratios ma tch observed values very well over a variety of instrumentation and so lar conditions. Since Mg VIII has a number of strong EUV lines, we exp ect that the spectra of this ion should prove useful for abundance stu dies and other plasma diagnostics.